Code-division multiple-access (CDMA) wireless communication is becoming the digital system of choice for upgrading or overlaying current analog wireless communication systems. CDMA provides many times the call handling capacity over analog cellular communications systems. See, for example, "An Overview Of The Application Of Code Division Multiple Access (CDMA) To Digital Cellular Systems And Personal Cellular Networks," May 21, 1992, available from Qualcomm, Inc., 10555 Sorrento Valley Road, San Diego, Calif. Briefly, a CDMA system transmits/receives voice or data in data packets between a mobile unit and a base station over a spread spectrum signal. A speech handler translates the CDMA data packets into and out of a digital 64 Kbps pulse-code modulated (PCM) stream for use in the standard connection-based digital public switch telephone network (PSTN). CDMA cellular communication is more fully described in documents from Qualcomm, Inc. ("The Wideband Spread Spectrum Digital Cellular System Dual Mode Mobile Station-based Station Compatibility Standard" and "CDMA Digital Common Error Interface Standard," revision 1.0, October, 1993).
As a CDMA mobile unit traverses a geographic area served by a CDMA wireless system, a call is handled by a progression of cell sites or sectors of cell sites, which keeps the call active in a predetermined region of the total geographical area. Changing from one sector or cell site to another is known in the art as a "handoff." One of the major advantages of CDMA is the ability to provide "soft" handoffs; that is, a handoff without a user-detectable disruption of communication. Such soft handoffs are facilitated by multiple cell site or sector transceivers being involved in the call at the same time during the handoff.
Turning to FIG. 1, a CDMA handoff is demonstrated. Each cell site (for example, cell site 1-1) provides voice communications with mobile units, such as 6, within its area, and performs several other tasks as well. A controlling cell site causes all mobile units it is serving to synchronize their internal clocks to the cell site's master clock. Such synchronization is required (see the standards cited above) in order to provide system functionality. One of the areas requiring clock synchronization is pilot signal reading. While mobile unit 6 is in cell 1 under control of cell site 1-1, cell site 1-1 sends a list of adjacent cells (the "neighbor list") to mobile unit 6 as possible candidate handoff cells. Cell site 1-1 then causes mobile unit 6 to check periodically signal strength of the pilot signals being broadcast by adjacent cell (or sectors).
Each cell site or sector broadcasts a pilot signal sequence, starting at a predetermined time offset. The mobile unit determines the signal strength of the pilot signal from the respective cell site or sector of the controlling cell site and the cell sites or sectors on the neighbor list. When mobile 6 detects that the signal strength mobile 6 is receiving of a pilot signal has exceeded a predetermined threshold, then the mobile unit reports the signal strengths for the cell sites on its neighbor list. A handoff may be requested by the cell site sector to a cell site with the strongest pilot signal. The call is then shared in a soft handoff mode between the original sector or cell site and the neighbor sector or cell site, until the signal strength from one of the sharing cell sites or sectors falls below another threshold. If a new cell site or sector becomes the controller, then the clock in the mobile is resynchronized and a new neighbor list is delivered. This handoff procedure is specified in the above references (also see IS 95 standard and U.S. Pat. Nos. 5,363,369, 5,434,853, 5,434,854, and 5,438,565), and described below.
One of the great advantages of CDMA is that, during handoff in a "soft" handoff mode, more than one cell site or sector is receiving content bearing packets. Multiple sets of packets are delivered to a central location (for example, MTSO 16), where a decision is made as to which packet has a stronger signal indication (the signal strength being a parameter encoded in each packet). MTSO 16 then chooses the packet with the stronger signal and sends it to public switched telephone network 18. Likewise, in the opposite direction, content from PSTN 18 arrives at MTSO 16, which, realizing that the packet is for mobile 6 in handoff mode, copies the packet and sends it to both cell sites or sectors involved in the handoff (in this example cell sites 1-1 and cell 3-3). Mobile 6 will receive both packets and translate the stronger packet into whatever content (audio or data) the user is expecting.
When a mobile unit reaches the end of a geographic coverage region, the system performs what is known in the art as a hard handoff. A hard handoff usually occurs when the connection between the mobile unit and the public switched telephone network changes (due to a change in frequency/operating mode, frame offset, etc.). In this scenario, the mobile is dropped from one system and added to another system as if a call were being initiated or, alternatively, a handoff may be prearranged with the new service provider through interservice provider protocol. Such hard handoffs are usually perceived by the user as an interruption in service and, if not carefully established, can result in loss of the telephone call.
Digital wireless systems (such as CDMA) are being deployed in urban areas first; primarily where analog systems are overloaded. However, large urban areas cause problems for CDMA system handoffs because signal strength measurements can vary rapidly as the mobile unit moves. For example, the mobile may move into a viaduct or bridge, or near a truck, building or other obstacle which interferes with microwave transmission/reception. Buildings and other objects blocking the most direct signal path between the cell sites and the mobile create "multipath" (which is the same problem that causes "ghosts" on broadcast television channels in most cities) caused by the signal from the mobile unit reflecting off of objects. Thus, the mobile may receive multiple signals for the same pilot, and, because of multipath interference, receive a "weaker" pilot signal (larger time offset) than actual distance between the mobile and all sites would normally indicate. In some instances, the CDMA system may be able to distinguish among the various multipath signals if the time between reception of the multiple paths exceeds the resolution bandwidth by the process of "rake review," as known in the art.
Thus, in areas where multipath is a problem, a CDMA mobile unit may detect a lower pilot signal from its currently serving cell site, even though it is fairly close to the cell site, and, as a result, a determination is made that a handoff is needed. At the same time, the mobile unit may have greater pilot signal strength indications for the neighbor list cell sites or sectors for the same reason. Such an inaccuracy may result in the call being handed off to a cell site or sector that is not the best candidate, even though its signal strength may indicate that it is the best candidate when the mobile moves again, the received signal strength may vary again, causing a further handoff initiation. As a result, calls may be dropped or lost.
Further, when a mobile unit reaches the edge of a CDMA coverage area, there may be (and frequently will be during the early stages of deployment) a problem with handoff to another system. These handoffs are known as "inter-carrier" handoffs (a handoff from one carrier's system to another's). If the other carrier is not a CDMA system operating on the same frequency as the first, then the CDMA system must perform a "blind" or "leap of faith" handoff wherein the mobile unit is instructed to change from one frequency to another, or from digital to analog, or both.
Therefore, a problem in the art is that calls may be lost due to a non-optional handoff or a handoff to an area that does not have a CDMA system, thus causing the call to be dropped.